Interpretive Summary: A group of fungi of the genus Fusarium is known to produce the toxin fusaric acid. This toxin was shown to be produced by most species of this genus, and has a wide spectrum of activity, including toxicity to plants and animals. In efforts to control the growth of the Fusarium fungi, several species of bacteria are used to protect various plants, especially corn. The objectives of this research were to determine how much fusaric acid is produced in corn, and then to determine if these concentrations were effective in preventing the growth of biocontrol bacteria. This research established for the first time that fusaric acid increased in concentrations in maize plants subjected to drought stress, compared to non-drought stressed plants. This research also established that the highly desirable endophytic species of bacteria was very sensitive to the in planta concentrations of fusaric acid, and that other bacterial species were either suppressed or insensitive. Thus, biocontrol bacteria intended for use to control the Fusarium species must be resistant to fusaric acid, which might be achieved by resistant mutants, either naturally occurring or experimentally induced.

Technical Abstract:
Fusaric acid was historically considered a nonspecific wilt toxin produced by species of Fusarium fungi. This toxin however, has been shown to inhibit the biosynthesis of the antimicrobial metabolites, as well as the growth of several bacterial species, some of which have biocontrol potential for reducing the growth of pathogenic and mycotoxic species of Fusarium. In this study the effects of fusaric acid on the growth of several strains of the bacterial endophyte Bacillus mojavensis, and its close relatives B. subtilis, B. amyloliquefaciens, B. atophaeus, B. licheniformis, B. vaillimortis, Paenibacillus lentimorbus, P. popilliae were determined. These strains were tested at the in planta concentrations determined in maize infected with a nonpathogenic isolate of Fusarium verticillioides. The concentrations of fusaric acid was determined in maize plants grown for 21 days under normal cultural conditions and under drought stress. The concentrations of fusaric acid, determined with HPLC, was higher in plants under drought stress than those grown under non-drought conditions. The growth of bacteria was measured under both concentrations indicated that the bacterial endophyte B. mojavensis, was very sensitive to both concentrations, which was toxic to this species. However, the effects of these two concentrations varied in that fusaric acid was toxic to some species, but in other it repressed growth. These data indicate that the in planta concentrations produced by symptomless growth of F. verticillioides is inhibitory to the growth and subsequent colonization of plants by bacteria sensitive to fusaric acid. Thus, under the low soil moisture Fusarium-infected maize might be highly competitive against fusaric acid sensitive biocontrol bacteria.